1. Technical Field
The present invention relates generally to wireless cellular communication systems; and more particularly to a cellular system in which an idle mobile station determines when it leaves a first cellular system and proceeds into a second cellular system and performs system selection to transition from the first cellular system to the second cellular system.
2. Related Art
Cellular wireless communication systems are generally known to include a plurality of base stations dispersed across a geographic service area. Each of the base stations includes at least one antenna and a base station transceiver system (BTS) and provides wireless service within a respective cell. The BTSs couple to base station controllers (BSCs) with each BSC serving a plurality of BTSs. The BSCs also couple to a mobile switching center (MSC) which interfaces to the Public Switched Telephone Network (PSTN) and other MSCs. Together, the BTSs, BSCs and the MSC form a wireless network which provides wireless coverage to mobile stations (MSs) operating within a respective service area.
Wireless communication systems operate according to various protocol standards. One particular protocol standard in place worldwide is the CDMA protocol standard. CDMA is a direct sequence spread spectrum system in which multiple spread spectrum signals are transmitted and received simultaneously over a common frequency band. In the CDMA system, each mobile station (MS) may be assigned a distinct Walsh code which identifies the signals transmitted to and received from the MS.
In an example of operation thereunder, forward link signals from a BTS to a first MS are spread with a first Walsh code and then transmitted where the process of transmission includes pseudo noise (PN) scrambling (spreading). Likewise, forward link signals transmitted from the BTS to the second MS are spread with the second Walsh code and then transmitted, perhaps concurrently with transmissions from the BTS to the first MS. The first MS""s receiver receives at its antenna all of the energy transmitted by the BTS. However, because Walsh code channels are orthogonal, after despreading the received signal with the first Walsh code, the despreader outputs all the energy intended for the first MS but, due to orthogonality loss, none of, or only a small fraction of the energy intended for the second, third, etc., MS. Likewise, the second MS despreads the received forward link signal with the second Walsh code to receive its intended forward link energy. Each of the MSs then operates upon the despread signal energy to extract data intended for the respective MS. The number of users accommodated on the forward link is limited by intra-cell interference due to orthogonality loss, inter-cell interference and other interference such as that due to thermal noise.
Because signals intended for other users of the CDMA system may appear as noise to other users due to orthogonality loss and because transmissions from other cells are not orthogonal to transmissions from a serving cell, CDMA capacity is interference limited. The number of users that can use the same spectrum and still have acceptable performance is determined by the total interference power that all of the users, taken as a whole, generate. Thus, the number of users that may be supported by each BTS on any frequency spectrum is limited.
In the CDMA cells at the border of the CDMA service region and analog AMPS service region, the coverage of the CDMA forward link overhead channels (e.g., Paging Channel, Pilot Signal, etc.) can be very large due to the near absence of out-of-cell interference. A CDMA mobile station in idle mode tends to cling to the Paging Channel as it travels beyond the Reverse Traffic Channel (RTCH) coverage limit of these cells. The area between the RTCH coverage limit and the Paging Channel coverage limit can be thought of as a xe2x80x9cDead Zonexe2x80x9d. Any attempt at call origination or page response in this area is unsuccessful due a high Frame Error Rate (FER) on the RTCH, yet the mobile station does not attempt to find another serving system due to the (reasonably) high quality of the Paging Channel. Since subscribers in the Dead Zone see on the mobile station display that the CDMA system is available, their perception is poor system performance.
For the CDMA/AMPS dual-mode mobile stations, this problem is a serious concern because, typically, the subscriber expects reliable service even beyond the CDMA coverage area. For the CDMA-only single-mode mobile stations, the subscribers typically do not expect such a large service area but they still expect system access reliability when the mobile station shows that CDMA is available. Note that this problem also arises within the CDMA service region when the first RF carrier is only partially overlaid by a second RF carrier (in a tiered fashion).
Several methods for mitigating this problem exist. For example, the power in the overhead channels can be reduced in order to xe2x80x9cconvincexe2x80x9d the mobile to give up the CDMA system closer to the cell site. In some cases this method is reliable but it does have some limitations. Since a mobile station timer, T30m, as defined in the TIA/EIA-95-B CDMA standard allows the mobile station to remain locked to a Paging Channel even if the mobile station receives only one good Paging Channel message in 3 seconds, the required reduction in overhead channel power could be large. This result can be an unacceptable loss in coverage close to the cell site, especially in buildings. Also, even after the mobile station leaves the current serving CDMA system/carrier, the ensuing system search could take a long time (dozens of seconds in some cases).
Controlling overhead channel power by using antenna down-tilt has exhibits performance similar to the above with the following drawbacks. First, there is a practical limit to the amount of mechanical down-tilt which can be applied to an antenna before the coverage pattern becomes too distorted. Second, mechanical down-tilt affects absolute traffic channel coverage as well as overhead channel coverage (though the problem of lost in-building coverage is reduced).
Finally, the Global Service Redirection message can be used to move idle traffic from the border CDMA cell/carrier to another system or carrier. However, this under-utilizes the hardware resources in the border cells and again can result in a long service outage period in which the mobile station tries to acquire the new server.
One possible solution to this problem is to reduce the T30m timer so that the mobile station will give up on the Paging Channel sooner (at a lower Maximum Error Rate [MER]) than it does currently. However, this could result in the mobile station beginning a complete system search after merely passing through a temporary coverage hole or area of high interference. This solution also fails to reduce the new system search time.
Thus, there is a need in the art for a method of operating a CDMA wireless communication system and a mobile station therein in which the mobile station recognizes when it is in the Dead Zone and performs system reselection but does not perform such system reselection when it is not in the Dead Zone.
Thus, to overcome the shortcomings of the prior systems, among other shortcomings, a wireless communication system and mobile station constructed according to the present invention employ meaningful thresholds to evaluate the quality of the transmissions from a serving base station to determine when the mobile station enters the Dead Zone adjacent a border cell of the system. Such thresholds not only consider the ratio of carrier energy to interference (Ec/Io) but also consider the energy of the carrier (Ec) itself. When the relevant measured (or calculated) carrier energy Ec and/or carrier energy to interference ratio Ec/Io are passed, the mobile station performs system reselection, attempting to acquire a different system/carrier.
The thresholds employed may be passed from the base station to the mobile station in the TIA/EIA-95-B Extended System Parameters Message, for example. Specifically, the parameters EC_IO_THRESH and EC_THRESH are used to indicate the EC/IO threshold and total receive power threshold, respectively, below which the mobile station should perform system re-selection. These parameters may be determined by the system operator using drive test data, by historical operating information by simulating the system or by other means. Alternatively, the system itself may determine the parameters based upon feedback received, such as the quality of reverse link transmissions from mobile stations operating in a respective cell. As is apparent, these thresholds may be similar in magnitude for border cells across the system. However, some variation will likely be encountered due to the operating characteristics of the base station serving the cell, the RF propagation characteristics within the cell and other factors which affect both forward link and reverse link performance.
According to the present invention, additional parameters may also be sent from the system to the mobile station relating to bordering systems and/or overlaying systems. These parameters may be passed in an Extended System Parameters Message, a Neighbor List Message, an Extended Neighbor List Message or a General Neighbor List Message, for example. These additional parameters suggest to the mobile station particular systems/carriers which the mobile station should try to acquire. Such systems/carriers information provide information regarding other CDMA systems and other analog systems, among other types of systems.
Moreover, other aspects of the present invention will become apparent with further reference to the drawings and specification which follow.